1. Field of the Invention
The present invention relates to techniques for protecting a copyright of content recorded on a writable recording medium on which a media number unique to the recording medium has been recorded.
2. Prior Art
A content distribution form of receiving content such as a movie film that is broadcast by digital broadcasting, recording the content on a recording medium such as an optical disk, and reproducing the content from the recording medium is becoming widespread in recent years. On the other hand, unauthorized distribution and reproduction of such content by copying the content through a personal computer and the like without license is also increasing.
To block unauthorized copying, a copyright protection system shown in FIG. 1 is conventionally known as a technique of restricting distribution and reproduction of content.
In FIG. 1, the copyright protection system is roughly made up of a recording device 1000 and a reproduction device 3000. The recording device 1000 encrypts content and records the encrypted content on a recording medium 2000. The reproduction device 3000 decrypts the encrypted content recorded on the recording medium 2000, and reproduces the decrypted content. The recording device 1000 includes a key encryption key computing unit 1001, a content key generating unit 1002, a content key encrypting unit 1003, a content storing unit 1004, and a content encrypting unit 1005. The reproduction device 3000 includes a key decryption key computing unit 3001, a content key decrypting unit 3002, a content key temporary storing unit 3003, a content decrypting unit 3004, and a digital AV processing unit 3005.
The recording medium 2000 is a recording medium such as an optical disk, and has a media number area 2001 in which a media number has been recorded. A media number is an identifier that is unique to each recording medium, and is written at the time of manufacturing the recording medium. The media number area 2001 is protected so that the media number recorded at the time of manufacturing cannot be rewritten thereafter.
The recording device 1000 acquires content from the outside, and stores the acquired content in the content storing unit 1004.
When the recording medium 2000 is connected to the recording device 1000, the content encrypting unit 1005 reads the content from the content storing unit 1004, and encrypts the content by using a content key. The content encrypting unit 1005 records the encrypted content in an encrypted content area 2003 of the recording medium 2000. Here, the content key is a random number generated by the content key generating unit 1002. The content key encrypting unit 1003 encrypts the content key by using a content key encryption key, and records the encrypted content key in an encrypted content key area 2002 of the recording medium 2000. Here, the content key encryption key is a key computed by the key encryption key computing unit 1001. The key encryption key computing unit 1001 computes the content key encryption key according to a hash function, by using a master key and the media number recorded in the media number area 2001. The master key is a key which is held commonly by the recording device 1000 and the reproduction device 3000 in secrecy from third parties.
FIG. 2 shows an inner computational mechanism of the key encryption key computing unit 1001.
The media number is input from a point A, and is encrypted by a DES encrypting unit 4000 using the master key held in a master key storing unit 4001 according to DES (Data Encryption Standard). After this, an exclusive-OR operation is performed on the encrypted media number and the media number by an exclusive-OR circuit 4002. The outcome is output from a point B as the content key encryption key.
On the other hand, when the recording medium 2000 is connected to the reproduction device 3000, the key decryption key computing unit 3001 reads the media number from the media number area 2001 of the recording medium 2000. The key decryption key computing unit 3001 performs the same computation as the key encryption key computing unit 1001 in the recording device 1000, to obtain a content key decryption key. Here, if the key decryption key computing unit 3001 uses the same master key and media number as the key encryption key computing unit 1001, the content key decryption key will end up being the same as the content key encryption key.
The content key decrypting unit 3002 reads the encrypted content key from the encrypted content key area 2002, and decrypts the encrypted content key by using the content key decryption key to obtain the content key. The content key decrypting unit 3002 temporarily stores the content key in the content key temporary storing unit 3003.
The content decrypting unit 3004 reads the encrypted content from the encrypted content area 2003, and decrypts the encrypted content by using the content key to obtain the content.
The digital AV processing unit 3005 converts the content to analog audio/video data, and outputs the converted data to external devices such as a speaker and a display.
Thus, the encrypted content key can be decrypted properly only if the reproduction device 3000 uses the same media number as that used for encrypting the content key.
In other words, if the reproduction device 3000 uses a media number which is different from the media number used for encrypting the content key, the encrypted content key cannot be decrypted properly.
Suppose an unauthorized party copies the encrypted content key and encrypted content recorded on the recording medium 2000, to another recording medium. Even if the reproduction device 3000 tries to reproduce this recording medium, the reproduction device 3000 cannot recover the correct content key from the encrypted content key copied on this recording medium, since a media number of this recording medium is different from that of the recording medium 2000.
Thus, the conventional copyright protection system enables the reproduction device 3000 to properly decrypt an encrypted content key of an original recording medium which has been recorded by the recording device 1000. On the other hand, the conventional copyright protection system makes it impossible for the reproduction device 3000 to properly decrypt an encrypted content key of a copy recording medium, thereby preventing content from being distributed by unauthorized copying.
The reproduction device 3000 reproduces a recording medium irrespective of whether the recording medium is an original or a copy. This being so, if the recording medium is a copy, the reproduction device 3000 will end up reproducing unintelligible data which is different from the original content, as video and audio.
A user who is using the recording medium without knowing it is a copy may suspect so, seeing that the reproduced video and audio are abnormal. However, abnormal reproduction can also be caused by other factors such as a malfunction of the device and an error of reading data from the recording medium. Therefore, even if an abnormal reproduction occurs, the user cannot determine right away that the recording medium is a copy.